The present invention relates to a capacitor discharge type ignition apparatus for an internal combustion engine.
A typical example of such an ignition apparatus for an internal combustion engine is diagrammatically shown in FIG. 4. The ignition apparatus illustrated includes a DC/DC converter 2 connected to a battery 1, a capacitor 3 connected to the DC/DC converter 2, an ignition coil 4 having a primary winding and a secondary winding having their one ends commonly connected to the capacitor 3, a signal generator 5 connected to the battery 1, a trigger circuit 6 connected to the battery 1 and the signal generator 5, and a thyristor 7 having an anode connected to a junction between the DC/DC converter 2 and the capacitor 3, a cathode connected to ground and a gate connected to the trigger circuit 6.
The battery 1 generates a voltage of 12 volts for example, and the DC/DC converter 2 boosts the output voltage of the battery 1 to a voltage of 400-500 volts for example.
In operation, the output voltage of the battery 1 is boosted to an appropriate level by the DC/DC converter 2 and supplied to the capacitor 3 to charge it.
The voltage of the thus charged capacitor 3 is discharged through the thyristor 7 when the thyristor 7 is turned on, i.e., at an ignition point of time. A discharging current flows from one end of the capacitor 3 through the thyristor 7 and the primary winding of the ignition coil 4 to the other end of the capacitor 3, so that a spark plug (not shown) connected to the secondary winding of the ignition coil 4 generates a spark for firing an air/fuel mixture in a cylinder.
With the known ignition apparatus as constructed above, the charged voltage of the capacitor 3 is discharged according to a time constant which is determined by the impedance of the primary winding. Accordingly, as the time constant decreases, the discharging becomes faster, so the rising of the output voltage developed at the secondary winding of the ignition coil 4 becomes quicker while making the time for generating a spark shorter. Conversely, as the time constant increases, the longer the sparking time becomes, so the rising of the secondary winding voltage becomes slower. As a result, it is difficult to achieve a quick rising of the secondary winding voltage and a long sparking time at the same time.